1. Field of the Invention
The present invention relates to subterranean well cementing operations, and more particularly, to improved well cement composition fluid loss control additives, well cement compositions containing the additives and methods of using the compositions.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in subterranean well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby strings of pipe such as casings and liners are cemented in well bores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of a well bore and the exterior surfaces of a pipe string disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein which supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore. Hydraulic cement compositions are also utilized in remedial cementing operations such as plugging highly permeable zones or fractures in well bores, plugging cracks or holes in pipe strings and the like.
Fluid loss control agents are used in well cement compositions to reduce fluid loss from the cement compositions to permeable formations or zones into or through which the cement compositions are pumped. In primary cementing, the loss of fluid, i.e., water, to permeable subterranean formations or zones can result in premature gelation of the cement compositions whereby bridging of the annular space between the permeable formation or zone and the pipe string being cemented prevents the cement compositions from being placed over the entire length of the annulus. A number of fluid loss control additives have been utilized in well cement compositions heretofore. A particularly useful cement composition fluid loss control additive is described in U.S. Pat. No. 4,555,269 issued to Rao et al. on Nov. 26, 1985 which is assigned to the assignee of this present invention. The fluid loss control additive is comprised of copolymers and copolymer salts of N,N-dimethylacrylamide and 2-acrylamido-2-methylpropane sulfonic acid having molecular weights of between about 75,000 and 300,000.
While the above well cement composition fluid loss control additive and other heretofore known fluid loss control additives have functioned successfully, there are continuing needs for improved well cement composition fluid loss control additives which bring about less fluid loss while maintaining static cement gel strength properties, well cement compositions including the additives and methods of using the compositions.
The present invention provides well cement composition fluid loss control additives, well cement compositions including the additives and methods of using the compositions which meet the needs described above and overcome the deficiencies of the prior art. The improved well cement composition fluid loss control additives are comprised of terpolymers having molecular weights in the range of from about 75,000 to about 300,000. The terpolymers are comprised of first, second and third monomers wherein the first monomer is selected from the group of 2-acrylamido-2-methylpropane sulfonic acid, 2-allyloxy-2-hydroxy-1-propane sulfonic acid and vinylsulfonic acid, the second monomer is selected from the group of N,N-dimethylacrylamide, acrylamide, N-vinylpyrrolidone, N-vinylacetamide and acrylonitrile and the third monomer is selected from the group of C6 to C22 dimethylaminopropylmethacrylamide bromide, chloride and iodide and C6 to C22 dimethylaminoethylmethacrylate bromide, chloride and iodide. A preferred such fluid loss control additive is comprised of a terpolymer of 2-acrylamido-2-methylpropane sulfonic acid, N,N-dimethylacrylamide and hexadecyldimethylaminopropylmethacrylamide bromide.
The improved cement compositions of this invention are basically comprised of a hydraulic cement, sufficient water to form a slurry and a terpolymer fluid loss control additive of this invention.
The methods of this invention for cementing a zone in a subterranean formation penetrated by a well bore are basically comprised of the steps of providing a cement composition of this invention as described above, placing the cement composition in the zone to be cemented and allowing the cement composition to set into an impermeable solid mass therein.
It is, therefore, an object of this invention to provide improved well cement fluid loss control additives, cement compositions containing the additives and methods of using the compositions.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.